
Reduction of tensor structure of loop diagrams using
the Passarino-Veltman (PV) method [1]}
based on Lorentz covariance of matrix elements and fractioning
of the denominators in loop integrals. For extensions of the alternative
PV methods up to six external legs see Ref. [2]};

Reduction of numerators containing virtual momenta by means
of derivatives acting on linear combinations of external
momenta [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]};

Reduction of tensor
to scalar integrals with shifted dimension [3]},
which further reduce to the loop integrals
in generic dimension [4]}, [5]};

using helicity methods in evaluating loop integrals [13]}, [14]};

Method based on integration by parts (IBP) reduction
is widely used for handling two and higher loops integrals [15]};

Methods based on decomposition of Lorentz vectors
in a parallel and orthogonal space in the context of one-loop
and multi-loop integrand reduction, and generalized
unitarity [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]};

Development of analytical and numerical methods for \(\varepsilon \) -expansion of
loop integrals (see, e.g., Refs. [24]}, [25]}).

